Electron beam amplification apparatus



y 1959 J. A. RICH 2,894,170

ELECTRON BEAM AMPLIFICATION APPARATUS Filed April 28, 1955 Q E lnvenzor g 554M l/azmat A. 0

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United State P tent ELECTRON BEAM AIVIPLIFICATION APPARATUS Joseph A. Rich, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Application April 28, 1955, Serial No. 504,519 4 Claims. (Cl. SIS-5.41)

My invention relates to an electron beam amplifier and more particularly to an improved method and apparatus for effecting the amplification of electric waves.

Electron beam amplifier devices, in which a slow wave structure, such as a helix, is utilized to establish a high frequency electric field for interaction (continuous in time and extended in space) with an electron beam projected through the helix, are well known. In these devices, the high frequency wave is transmitted along the helix at approximately the speed of light. The phase velocity of the high frequency wave in the direction of the axis of the helix is much less, depending particu larly upon the pitch of the helix. In operation, the average velocity of the beam is adjusted to approximate the phase velocity of the wave in an axial direction.

More recently, electron beam amplifiers have been proposed which utilize the amplification produced in the beam itself where the beam has a suitable recurring variation in space charge density along its length. The present invention relates to improved amplifiers of this type in which the beam is maintained substantially uniform in diameter and is subjected: to a periodic and gradually varying voltage along its axis. More specifically, the beam is subjected to a strong axial magnetic focusing field which maintains the beams at substantially constant diameter throughout its length and subjected to a voltage component which varies cyclically, preferably sinusoidally, with distance along the axis of the beam.

It is an object of my invention to provide a new and improved method and apparatus for amplifying an electric wave by means of an electron beam. 7

It is a further object of my invention to provide a new and improved method and apparatus for producing a sinusoidal variation in space charge density along the length of the beam of an electron beam amplifier.

Further objects and advantages will become apparent as the following description proceeds, reference being had to the accompanying drawing and its scope will be pointed out in the appended claims. In the drawing, Fig. 1 is an elevational view in section of an electron beam amplifier embodying my invention and operable in accordance with the method of my invention. Figs. 2 and 3 are respectively elevational views in section of moditied forms of my invention. Fig. 4 is a graph to illustrate the voltage variation with distance along the axis of the amplifier of my invention.

Referring now to the drawing, I have shown my invention embodied in an electron beam amplifier including a cathode 10 and a generally cylindrical hollow collecting electrode 11 supported at opposite ends of the device for emitting and collecting respectively the eleccan; of a relatively long electron beam shown diagrammatically at 10'. As will become apparent as the description proceeds, these electrodes also form a part of the input and output circuits respectively of the amplifying device. Referring now to the cathode or input end of the device, the cathode includes a circular emitting ice surface 12 supported from the end of a cylinder 13 within which is supported a suitable resistance heater element 15. Spaced from the emitting surface 12 in an axial direction is a control grid 16 supported from a generally annular washer 17, which, in the particular embodiment illustrated, is formed as an outwardly extending flange of a cylinder 18 which surrounds the cathode cylinder 13 and cooperates therewith to provide a concentric input resonator. The cylinder 18 is joined to the cathode cylinden 13 in vacuum-tight relation by means of an annular sealingmember 19. The resonant frequency of the input resonator may be adjusted suitably by the positioning of" a slidable annular end wall 20. Excitation. of the resonator is eifected by an input conductor 21, shown as extending through the end Wall 20 and coupled to the resonator by an inductive loop 22. The conductor 21 may be suitably shielded by a surrounding concentric conductor (not shown). As illustrated, the input resonator and cathode are supported from an annular insulating disc 23 by means of the flange 17 which is bonded to one side thereof. An accelerating grid 24 is supported from the opposite side of the insulator by means of a conducting annular disc 25.

At the opposite end of the discharge device, the collec'ting electrode 11 forms a part of a hollow resonator including a generally hollow cylindrical conducting member 26 which is bonded at the outer surface of one end wall 27 thereof to an annular insulator 28. vThe end wall is centrally apertured to provide .an opening in alignment with a reentrant .portion 11 of the collector electrode 11 to provide for the passage of the electron beam through the resonator. The openings in the end wall 27 and in the collectorelectrode are preferably closed with .30 inductively coupled to the resonator by a loop 31.

The central section of the electron beam device between the supporting washer 25 of the accelerating grid 24 at the input end and the insulating'disc 28 at the output endis the amplification section of the tube and is designed to subject the beam to a voltage which varies substantially sinusoidally with distance along the tube axis. The central portion comprises a succession of annular conducting rings 33 and 34 which are mutually insulated by means of interposed insulating discs 35. In the particular form illustrated, these electrodes are catenoids; that is, they each have the shape of a surface of revolution of a catenary rotated about the axis of the device. In other words, each electrode has a diameter which is a minimum at the middle thereof and which increases gradually toward each end. It is believed apparent that the shape of the electrodes described above will provide a voltage at the axis of the tube which is a maximum at the middle of the electrode and gradually decreases toward the ends of the electrode. If alternate electrodes are maintained respectively at direct current voltages above and below a reference voltage, such as the average voltage of the beam, the voltage variation along the axis of the tube will be substantially sinusoidal, reaching maximum values (with respect to the reference voltage) where the electrodes are of minimum diameter and having zero value in the plane of the insulator. i

In the particular embodiment illustrated, the output resonator and collector are operated at ground and the cathode accordingly is operated at a high negative voltage. For the purposes of supplying this voltage, I have illustrated a battery '36 with the negative terminal connected with the cathode cylinder 13 and with a plate near the positive end connected to ground. The output resonator is also grounded as illustrated at 38. Alternate electrodes 33 are connected together and to a plate 40 of the battery 36 more positive than the grounded plate by means of a conductor 41. In a similar way, electrodes 34 are connected together and to a plate 42 of the battery 36 more negative than the grounded plate by means of a conductor 43.

The electron beam is maintained substantially uniform in cross section throughout its length. For this purpose, I provide an electromagnet illustrated schematically at 44 for producing a strong axially extending focusing field within the discharge device. The magnet extends essentially throughout the length of the beam terminating in the region of the grid 24 at one end and the grid 29 at the other end. In the operation of the device, a magnetic field of approximately 1500 gauss is suitable for accomplishing the desired collimation. It should be noted here that the metal parts, particularly the electrodes 33 and 34, are of non-magnetic material, such as copper or non-magnetic stainless steel. The voltage of the two sets of electrodes which surround the beam are preferably maintained at equal direct current voltages above and below the voltage of the beam by an amount which may be in the order of 10% of the beam voltage. For example, these voltages may be in the order of 100-]- and 100 with an average beam voltage of 1000 volts. It should be noted also that the beam, shown at 10', has a relatively small diameter compared to the inner diameter of these electrodes so that it is subjected essentially to the voltage at the center of these electrodes and at the axis of the tube.

In Fig. 4 is illustrated the voltage variation with distance along the axis of the tube. This voltage variation is approximately sinusoidal with the voltage maxirna and minima occurring opposite the regions of minimum diameter of adjacent electrodes and the points of zero voltage or voltage equal to the average beam voltage occurring opposite the insulating spacers 35. With this arrangement and without any input signal impressed on the input circuit, the beam undergoes alternate accelerations and decelerations corresponding to the voltage variation along the axis resulting in cyclic variations in space charge density. This recurring gradual change in space charge density produces amplification of any signal impressed on the beam by means of the input circuit previously described. The electron beam is projected across the gap between the grids 29 and 30 to excite the output resonator 26 in accordance with the amplified signal.

In the construction and operation of devices embodying my invention, it is necessary to correlate the physical dimensions and spacing of the electrodes 33 and 34 in an axial direction with what is termed the plasma wave length. It is apparent that the width and spacing of the electrodes in an axial direction determines the periodicity of the voltage along the axis of the tube and as a result the periodicity of the space charge density. Also the plasma wave length is dependent upon the average beam velocity n and the average charge density p In other words, the axial length of the electrodes and the spacing therebetween is correlated with the average charge density of the beam and the mean direct current voltage of the beam. For amplification in the electron beam, it can be shown that the operating conditions should satisfy the following equation:

sponding to the charge density variation in the beam (dependent upon the physical dimensions of the electrodes and spacing in an axial direction) and Substituting in Equation 1 the value of m from Equation 3, we have Since k equals r 7\ where x is the wave lengthrof the space charge variation along the beam, Equation 4 becomes For the case where n=1, which is the case of practical significance, Equation 5 may be reduced to From the above, it is apparent that A and accordingly the axial width and spacing of the electrodes 33 and 35 is dependent upon the average direct current beam velocity u and the average charge density p in accordance with the relationship of Equation 6.

Equation 1 also gives A in terms of A. By substituting n=1,

and

4 1 u at we have )t 2 21r 2 (A101) Solving Equation 7 for a we have k =2 indicating that amplification is obtained when the plasma wavelength is twice the wavelength of the periodically varying space charge density (along the axis).

While I have shown and described a particular form of input and output circuit for impressing a signal on the beam and utilizing the amplified modulated beam, it will be apparent that other forms of circuits may be coupled to the beam for impressing a signal thereon and removing the energy therefrom. For example, these circuits may both be in the form of gaps of the type normally used in klystrons or they may be in the form of input and output helices, if desired. Regardless of the particular form of input and output circuits used, the present invention involves the amplification of the signal impressed on the beam by means of the cyclically varying space charge density along the length of the electron beam resulting from the recurring gradual change in velocity of the beam produced by the two sets of electrodes operated at direct current voltages respectively above and below the average direct current voltage of the beam.

In Fig. 2 I have shown a modification of my invention in which the electrodes. 33 and 34 of Fig. 1 have been replaced by a plurality of hollow cylindrical rings with alternate rings connected together in one group and the remaining rings connected together in the second group as described in connection with the electrodes 33 and 34 of Fig. 1. The input and output ends of the device shown in Fig. 2 are the same as those previously described in connection with Fig. 1 and the same reference numerals are employed to designate corresponding parts. In the modification shown in Fig. 2, the electrodes themselves do not form the envelope wall which is provided by a cylindrical non-magnetic envelope member 46 terminating in flanges sealed to the inner surfaces of the insulating Washers 23 and 28 respectively. The washer 48 corresponds generally to the washer 25 of Fig. 1 and supports the accelerating grid 24. The electrodes 44 and 45 are shown schematically as supported from the envelope wall 46 by means of conductor 49 extending through insulating sealing member 50. It will be readily appreciated that additional mechanical support may be provided for these electrodes, if desired.

In accordance with my invention, the ring-like electrodes 44 and 45 are spaced axially from one another by a distance which is small compared with the radius of the electrodes. The electrodes are also, preferably, short in an axial direction compared with the radius thereof so that the voltage produced along the axis of the tube is essentially the same as illustrated in Fig. 4 and described in connection with the operation of Fig. 1.

It is believed apparent that, within limits, the exact slope of the voltage produced at the axis of the tube may be varied by the dimensions of the electrodes 44 and 45 and the spaces therebetween. It is possible to approximate the same voltage variation along the axis of the tube as that described in connection with the specially shaped electrodes of Fig. 1.

In Fig. 3 I have shown a still further embodiment of my invention in which the voltage variation along the axis of the tube produced by a bifilar arrangement of two helices which in general produce a voltage along the axis essentially the same as that produced by the two sets of rings 44 and 45 of Fig. 2. As illustrated the two helices 50 and 51 are provided respectively with axially extending end portions 52 and 53 which are received in suitable recesses formed in the insulators 23' and 28'. The end flanges 47 and 48' are provided with openings in alignment with the recesses in the washers 23' and 28' to permit the passage therethrough of the ends 52 and 53 of the helices with suitable clearance to insulate the difference in operating voltages involved for these parts.

While I have shown and described a particular embodiment of my invention, it will be apparent to those skilled in the art that modifications may be made without departing from my invention and I, therefore, aim by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A high frequency electron beam amplifier comprising spaced electrodes and means for impressing a direct current voltage diiference between said electrodes to establish an elongated electron beam therebetween, input circuit means coupled to said beam near one end thereof, a plurality of ring-like electrodes surrounding said beam and supported side by side in mutually insulated relation along the length of said beam, the width of said electrodes and the spacing between adjacent electrodes being small compared to the diameter thereof, means energizing alternate ring-like electrodes with voltages above and below the direct current voltage of said beam to provide a voltage having substantially sinusoidal variation with distance along said beam, means for producing a magnetic field in a direction parallel to the direction of said beam to maintain said beam focused and of substantially uniform cross-section along its length in the presence of said substantially sinusoidal voltage variation, and output circuit means coupled to said beam near the opposite end thereof.

2. A high frequency electron beam amplifier comprising spaced electrodes and means for impressing a direct current voltage difierence between said electrodes to establish an elongated electron beam therebetween, input circuit means coupled to said beam near one end thereof, a plurality of ring-like electrodes surrounding said beam and supported side by side in closely spaced mutually insulated relation along the length of said beam, each of said electrodes having a diameter which is minimum at the middle thereof and which increases gradually toward each end, means energizing alternate electrodes with a direct current voltage respectively above and below the average direct current voltage of said beam to provide at the center of said beam a substantially sinusoidal voltage variation with distance along said beam, means for producing a magnetic field in a direction parallel to the direction of said beam to maintain said beam focused and of substantially uniform cross-section along its length in the presence of said substantially sinusoidal voltage variation, and output circuit means coupled to said beam near the opposite end thereof.

3. A high frequency electron beam amplifier comprising spaced electrodes and means for impressing a direct current voltage difference between said electrodes to establish an elongated electron beam therebetween, input circuit means coupled to said beam near one end thereof, a pair of bifilar wound helical electrodes surrounding said beam and supported in mutually insulated relation, the spacing between turns of said electrodes being small compared to the diameter thereof, means energizing said helical electrodes respectively with direct current voltages above and below the average direct current voltage of said beam to provide at the center of said beam substantially sinusoidal voltage variation with distance along said beam, means for producing a magnetic field in a direction parallel to the direction of said beam to maintain said beam focused and of substantially uniform crosssection along its length in the presence of said substantially sinusoidal voltage variation, and output circuit means coupled to said beam near the opposite end thereof.

4. A high frequency electron beam amplifier comprising spaced electrodes, means for impressing a direct voltage current difference between said electrodes to establist an elongated electron beam therebetween, input circuit means coupled to the beam near one end thereof, a. plurality of conductors encircling the beam along its length, means supporting adjacent conductors in mutually insulated and spaced relation, the width of said conductors and the spacing between adjacent conductors being small compared to the diameter thereof, means energizing alternate conductors respectively with voltages above and below the direct current voltage of the beam to provide a voltage having substantially sinusoidal variation with distance along the axis of the beam, means for producing a magnetic field in a direction parallel to the direction of said beam to maintain said beam focused and of substantially uniform cross-section along its length in the presence of said substantially sinusoidal voltage variation, and output circuit means coupled to said beam near the opposite end thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,190,511 Cage Feb. 13, 1940 2,489,082 De Forest Nov. 22, 1949 2,538,267 Pierce et a1 Jan. 16, 1951 2,548,118 Morton et al Apr. 10, 1951 2,725,499 Field Nov. 29, 1955 

